Question: Which of these numbers is composite? ${3,\ 11,\ 17,\ 39,\ 79}$
Answer: A composite number is a number that has more than two factors (including 1 and itself). 3, 11, 17, and 79 each have only two factors. The factors of 39 are 1, 3, 13, and 39. Thus, 39 is the composite number.